A. Field of the Invention
The present invention relates to a method for producing a perpendicular magnetic recording medium used for a magnetic recording apparatus. In particular, the present invention relates to a perpendicular magnetic recording medium that has a high recording density, a superior read-write characteristic, that is written to easily, and that has high thermal stability.
B. Description of the Related Art
Recently, a magnetic recording apparatus having a larger capacity and a smaller size has been desired as a memory device for a personal computer or a workstation. In order to achieve this, a magnetic recording medium must have a higher recording density. One of the magnetic recording methods in present use is the in-plane magnetic recording method (or a longitudinal magnetic recording method) which has an easy magnetization axis in a direction parallel to a magnetic recording medium surface. In order to improve recording density in the in-plane magnetic recording method, it is required to reduce the product of the residual magnetization (Br) of a magnetic film of a magnetic recording medium and a magnetic layer thickness (t). Coercivity (Hc) also must be increased. Thus, an attempt has been made to reduce the film thickness of a magnetic film to control the grain diameter of magnetic crystals. However, the in-plane magnetic recording method is disadvantageous in that a demagnetizing field is increased in accordance with the shortening of bit length to cause the reduction of the residual flux density, thus reducing the reproduction output. The in-plane magnetic recording method also has a problem with respect to thermal stability, which is brought about by the smaller magnetic crystal grains or thinner film. Therefore, it appears to be technically difficult to use this method to provide a magnetic recording medium having a higher density.
On the other hand, a perpendicular magnetic recording method has been explored as a means for solving the above-described problems and for improving the surface recording density. In the perpendicular magnetic recording method, a magnetic recording medium is provided in which the easy magnetization axis of a magnetic film is perpendicular to a substrate face, and neighboring magnetizations in a magnetization transition region are not opposed to each other. Thus, even when bit length is reduced, the magnetization is stable and the reduction in magnetic flux as in the in-plane magnetic recording is small. Therefore, this medium is appropriate as a high density magnetic recording medium.
As described above, the perpendicular magnetic recording medium is more advantageous as a high density magnetic recording medium than the in-plane magnetic recording medium. However, in the case of a perpendicular magnetic recording medium, segregation of non-magnetic elements at a magnetic crystal grain boundary is not satisfactorily performed, causing a problem in which magnetic interaction among magnetic crystal grains is increased, giving rise to increased noise. Therefore, conventionally an attempt has been made to improve the layer structure of the perpendicular magnetic recording medium, in order to achieve a high density magnetic recording or the reduction of the medium noise.
As a known structure of a perpendicular magnetic recording medium, a bilaminar perpendicular magnetic recording medium has been disclosed in which a non-magnetic substrate consisting of aluminum, glass or the like, for example, has an overlying soft magnetic backing layer on which an underlayer for the perpendicular magnetic layer is provided. The underlayer has thereon a perpendicular magnetic layer and a protection layer. As a perpendicular magnetic layer, many perpendicular magnetic layers have been examined, including a perpendicular magnetic layer consisting of a Co-base alloy, e.g., CoCr, CoCrTa or CoCrPt; a multilayer lamination perpendicular magnetic layer in which a Co layer and a Pt layer or a Co layer and a Pd layer are alternately laminated (hereinafter simply referred to as Co/Pt, Co/Pd or the like); an amorphous perpendicular magnetic layer, e.g., TbCo or TbFeCo; and, recently, a perpendicular magnetic layer including an oxide, e.g., CoPtCrO or CoPtCr—SiO2 (see Japanese Patent Laid-Open Publication No. 2003-178413, for example). When a perpendicular magnetic layer includes oxide, a non-magnetic substance (oxide) is segregated at the magnetic crystal grain boundary to provide a superior perpendicular magnetic recording medium that has a small magnetic crystal grain diameter and that has a small magnetic exchange interaction among magnetic crystal grains.
Co/Pt and Co/Pd in particular have been actively researched as a future high recording density medium because they have a high perpendicular magnetic anisotropy constant (Ku) and thus have a high thermal stability and a high coercivity, and a squareness ratio 1.0 is easily obtainable (see Japanese Patent Application No. 2003-206091, for example). On the other hand, a magnetic recording head for writing information to a magnetic recording medium has a limitation on the magnetic field that may be generated due to the writing process, and thus it has difficulty in writing when the magnetic recording medium has a high anisotropic magnetic field (Hk). For a conventional magnetic recording medium, the increase in Ku causes an increase in Hk, thus making it difficult to provide both thermal stability and ease of writing by the recording head.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.